The invention relates to a thermosetting coating material which comprises a carboxyl-functional polyester or a polyacrylate and a specific selected cross-linking agent, a masked β-hydroxyalkylamide.
Powder coatings which contain carboxyl-functional polyesters and β-hydroxyalkylamides have already been known for a long time as bonding agents.
U.S. Pat. No. 4,801,680 describes powder coatings based on carboxyl-functional polyesters and β-hydroxyalkylamides, the polyester having a Tg within the range of 30° C. to 85° C. and an acid value of approximately 20 to 85 and the equivalent ratio of β-hydroxyalkylamide equivalents to carboxylic acid equivalents is within the range of 0.6 to 1.6:1.
Toxicologically safe powder coatings based on carboxyl-functional polyesters and/or carboxyl-functional polyacrylates and β-hydroxyalkylamides demonstrate very good exterior stability and are hence suitable for use outdoors. The β-hydroxyalkylamides used should contain at least two β-hydroxyalkylamide groups per molecule, i.e. be difunctional.
However, today higher-functional β-hydroxyalkylamides are used predominantly, such as for example the tetrafunctional β-hydroxyalkylamides bis[N,N′-di(β-hydroxyethyl)]-adipamide and bis N,N′-di(β-hydroxypropyl)]-adipamide.
Powder coatings which contain higher-functional (tetra or more) β-hydroxyalkylamides as hardeners still have certain disadvantages with respect to surface defects. Problems cause a partly still inadequate flow and inadequate degassing properties in the case of greater layer thickness.
U.S. Pat. No. 5,216,090 describes powder coatings based on carboxyl-functional polyesters and/or carboxyl-functional polyacrylates and mixtures of difunctional and higher-functional β-hydroxyalkylamides which clearly demonstrate improved flow control and degassing properties.